Hygrometer electrolytic cell

ABSTRACT

AN HYGROMETER ELECTROLYTIC CELL HAVING AS AN ELECTROLYTE AN ANHYDRIDE FORMED FROM AN ACID OF AN ELEMENT SELECTED FROM THE GROUP CONSISTING OF ALUMINUM, BORON, GROUP IV-A ELEMENTS AND GROUP V-A ELEMENTS. THE ACID HAS BLOCKED POLYMERIZATION SITES AND CONTAINS NOT MORE THAN THE EQUIVALENT OF 19 CARBON ATOMS.

Dec. 28, 1971 F. B. KUFFER I ELECTROLYTIC CELL HYGROMETER Filed March10, 1970 FIG. I

IN VIZNTOR.

FIG. 2

FERNAND B. KUFFER ATTORNEY United States Patent 3,630,875 HYGROMETERELECTROLYTIC CELL Fenland B. Kufier, Brea, Califi, assignor to BeckmanInstruments, Inc. Filed Mar. 10, 1970, Ser. No. 18,263 Int. Cl. G01n25/56 US. Cl. 204-195 15 Claims ABSTRACT OF THE DISCLOSURE An hygrometerelectrolytic cell having as an electrolyte an anhydride formed from anacid of an element selected from the group consisting of aluminum,boron, Group IV-A elements and Group V-A elements. The acid has blockedpolymerization sites and contains not more than the equivalent of 19carbon atoms.

BACKGROUND OF THE INVENTION The instant invention relates toelectrolytic cells and more specifically to electrolytic hygrometercells. It is particularly applicable to the type hygrometer celldescribed in US. Pat. No. 2,830,945, and commonly known as a Keidelcell.

Electrolytic hygrometers, particularly hygrometers of the Keidel type,have been widely used for determining the moisture contents of fluidstreams in industrial processes in which the presence of even minutepercentages of moisture is of great significance. This type hygrometerhas numerous advantages over other moisture determining devices, as itis quite selective to water, has a fairly rapid speed of response, andis completely quantitative over Wide ranges of moisture concentration,thereby eleminating the need for frequent calibration and standardsamples.

Electrolytic hygrometers of the Keidel type depend for their operationon the relationship between the amount of water present in a hygroscopicsubstance and the amount of current necessary to electrolyze it. Theactive elements generally comprise a pair of electrode wires, forexample of platinum, or rhodium, frequently partially embedded in asupporting tubular jacket. The inter-electrode space is coated with ahygroscopic substance such as phosphoric acid which, when dried, becomesmetaphosphoric acid, HPO The gas or vapor to be tested is passed overthe hygroscopic substance which absorbs the moisture present in the gasor vapor and becomes conductive. An electrolytic current is then passedbetween the two conductors and the water is electrolyzed to hydrogen andoxygen. The amount of current necessary to completely electrolyze theWater is, of course, a measure of the moisture content of the fluidbeing tested.

The normal response time for a cell of the type generally describedabove for a change in moisture level from near 0% humidity to near 100%relative humidity is about minutes for a properly Operating cell.Frequently it would be \desired to have a faster response time. Anexample of when a faster response time would be desired is if thehygrometer electrolytic cell Were being used to provide data for thecontrol of a process. Knowing how much Water vapor was in the processstream ten minutes ago simply is not good enough. A more rapid responsetime is desired. Another example is the measurement of very low waterconcentrations, e.g. below 50 parts per million.

SUMMARY OF THE INVENTION It is an object of the instant invention toprovide an electrolytic cell having an electrolyte which will result ina much faster response time in the cell from changes in waterconcentration in the gas or fluid sample stream. The electrolyteadvantageously is closely related to the ice BRIEF DESCRIPTION OF THEDRAWINGS FIG. 1 is a graphical presentation of an electrolytic cellemploying a conventional electrolyte.

FIG. 2 is a graphical presentation of the response time of a cellsimilar to that of FIG. 1, but employing an electrolyte according to theinstant invention.

DETAILED DESCRIPTION A general description of the operation of anelectrolytic cell of the Keidel type may be found above. The principallimitation on response time of the cell seems to be the chemicalreactions involved. The reactions are commonly stated as follows:

P205 313 0 2H3P04 ZHIQPOJ 311 0 P 05 (iii) These reactions are idealizedreactions and do not describe What actually occurs under the operatingconditions of the cell. The last two reactions under the conditions ofthe cell may be more accurately represented as:

The reverse reaction is asymptotic to HPO The asymptotic polymerizationreaction creates the speed problem, especially at low levels of water.Thus, under cell conditions the reaction more accurately is:

2H PO H P O +H O (v) 4 5 5 2 10 4 13 in a polymerization reaction to2HPO +the other water molecule (vii) As the reaction occurs, the rate ofreaction slows due to the glass-like form of the product of the reactionand the resultant limited availability and mobility of the H and OH tothe electrodes. Thus, it is apparent that the rate of reaction can besubstantially increased if it is possible to stop the polymerizationreaction and prevent the molecule from getting too big. Advantageouslythe reaction should be controlled so that it proceeds from the monomerto the dimer and then back again.

The instant invention prevents the polymerization reaction fromoccurring by utilizing an acid having blocked polymerization sites. Thepolymerization reaction is prevented and at the same time hygroscopicityof the material is maintained. For example, if the phosphoric acid hasorganic groups substituted for the hydroxy groups the polymerizationsites are blocked.

polymerization sites is potentially useful in the cell. However, theefficiency of the cell involves mols of material per mol of water. Thusthe molecular weight of the material being used for the electrolyteaffects the ratio and a higher molecular weight electrolyte decreasesthe chemical efficiency of the cell. Also, the molecules become moreunwieldy with increasing molecular weight. Therefore, the invention isdesirably limited to the use of acids containing not more than theequivalent of 19 carbon atoms with the core atom, in this casephosphorous, being counted as an equivalent carbon atom. When heavierele ments are used for the core atom, then the total number ofequivalent carbon atoms (i.e. the actual carbon atoms and the core atom)will desirably be lower than 19, as is obvious to those skilled in theart.

Depending upon the cell materials and the particular electrolyte to beused in the cell, the material may be applied in either the acid oranhydride form. For convenience, the anhydride shall be referred to asthe electrolyte.

As an example of the instant invention, dimethyl phosphinic acid wasprepared according to the following reactions:

[( m lz 2 2 a)z 1 1 Using a 2 liter round bottom flask fitted with amechanical stirrer, a reflux condenser with drying tube, and a pressureequalizing addition funnel, and which was immersed in a cold bath at -12C. F.), 104 grams of chicphosphoryl chloride was added dropwise withstirring in two hours to 2 mols of methyl magnesium chloride and 750milliliters of anhydrous ether. The reaction mixture was maintained at 8to 3 C. (17-26 F.) internal temperature during the addition. After theaddition the cooling bath was slowly lowered away from the flask over atwo hour period with continuous stirring. The reaction mixture wasallowed to warm up to room temperature and slowly poured into a mixtureof 200 milliliters of concentrated sulphuric acid and 2 kilograms ofcrushed ice. After the addition was complete the mixture was allowed tostand one hour with occasional stirring. The mixture was filtered andthe white solid washed with water and a small amount of ether and vacuumdried over night. The solid was recrystallized from a hot mixture of1--1 ethanol-benzene and air dried. The intermediate [(CH PS] had amelting point of 2295-2305 C. (445447 F.) and was added to 60milliliters of carbon tetrachloride in a 300 milliliter flask andbrought to reflux. Twenty-five milliliters of 30% hydrogen peroxide wasthen added slowly with stirring. The mixture was refluxed an additionaltwo hours with stirring, cooled,

and filtered with the filtrate separating into two phases.

The aqueous phase was dried in vacuum over phosphorous pentoxide and theproduct recrystallized twice from hot benzene. The yield was 3.9 gramsof material melting at 88.5 to 915 C. (l91l97 F. This material, in theform of a 20% by weight solution, was then used to coat three cells ofthe Keidel type utilizing the technique described in U.S. Pat. 3,240,693to Douglas B. Gardner.

The cell was abruptly exposed to about a relative humidity vapor. Asshown in FIG. 1, a conventional electrolytic cell was slow to sense(zone X) and responded relatively slowly (zone Y) to the sudden changein humidity. A typical response time would be about two and a halfminutes.

Referring now to FIG. 2 it may be seen that the cell containing theelectrolyte according to the instant invention was quick to sense (zoneX) and responded quite rapidly (zone Y) to the large step change inhumidity. In fact, the response time was less than thirty seconds.

Although in the above example R and R were both methyl groups, it willbe appreciated the invention is not limited thereto. Any suitableorganic group can be used to block the polymerization sites, such asalkyl groups, aryl groups, or substituted alkyl, or substituted arylgroups. R and R may be either the same or different. Similarly thematerial is not limited to dimethyl phosphinic acid. Practically anyanhydride having blocked polymerization sites and containing not morethan the equivalent of about 19 carbon atoms can be used effectively inthe practice of the instant invention. Thus, the anhydride can be formedfrom an acid of an element selected from the group consisting ofaluminum, boron, Group IVA elements, and Group VA elements, so long asthe polymerization sites are blocked and the molecule contains not morethan the equivalent of 19 carbon atoms. When the Group V-A compounds areused, the blocked acid has the following structural formula:

u R-X-OH and the anhydride:

i it R I O2| R R R in which R and R are organic groups, H is hydrogen, 0is oxygen, and X is selected from the group consisting of nitrogen,phosphorous, arsenic, antimony, and bismuth. When the material is ablocked acid of a Group IVA element, the blocked acid has the followingstructural formula:

0 l .OH it and the anhydride:

0 0 that 1'. 1'.

in which R is an organic group, O is oxygen, H is hydrogen, and Y isselected from the group consisting of carbon, silicon, germanium, tin,and lead.

When aluminum or boron are used as the core element, the structure inits blocked form is more accurately that of a hydroxide than of an acidbut may still be referred to as blocked acid. Thus, the blocked acid ofthese two elements would have the following structural formula:

in which R and R are organic groups, H is hydrogen, 0 is oxygen, and Zis selected from the group consisting of boron and aluminum.

Typical anhydrides useful as electrolyte materials according to theinstant invention are acetic anhydride, acetic propionic anhydride,n-valeric anhydride, isovaleric anhydride, dichloroacetic anhydride,caproic anhydride, n-heptoic anhydride, benzoic anhydride, lauricanhydride, chloroacetic anhydride, itaconoic anhydride, stearicanhydride, m-toluic anhydride, phenylacetic anhydride, o-chlorobenzoicanhydride, m-chlorobenzoic anhydride, p-toluic anhydride,4-nitrophthalic anhydride, cinnamic anhydride, phthalic anhydride,o-nitrobenzoic anhydride, a-naphthoic anhydride, m-nitrobenzoicanhydride, 3-nitrophthalic anhydride, 1,2-naphthalic anhydride,p-nitrobenzoic anhydride, p-chlorobenzoic anhydride, d-camphoricanhydride, 2,3-naphthalic anhydride, 1,8-naphthalic anhydride,tetrabromophthalic anhydride, and tetraiodophthalic anhydride. Otheruseful anhydrides include dimethyl phosphinic anhydride, methyl propylphosphinic anhydride, ethyl hexyl phosphinic anhydride, dioctylphosphinic anhydride, and octyl decyl phosphinic anhydride. All theelements mentioned above make blocked acids and blocked anhydridessimilar to those of carbon and phosphorous. If a blocked acid ofnitrogen is being used, it is desirable to keep the organic groups largeenough so that the material is either a liquid or solid rather than agas.

While there have been shown and described hereinabove certainembodiments of this invention, it is to be understood that the inventionis not limited thereto and that various changes, alterations, andmodifications can be made thereto without departing from the spirit andscope thereof as defined in the claims.

What is claimed is:

1. In a hygrometer electrolytic cell adapted for the determination ofwater content of a gas or liquid sample wherein the cell comprises anelectrolyte material which is hygroscopic to water and wherein means areprovided for electrolyzing the water absorbed by the electrolyte toprovide an indication of the amount of water in the gas or liquidsample, the improvement wherein the electrolyte material is an anhydrideformed from an acid of an element selected from the group consisting ofaluminum, boron, Group IV-A elements, and Group V-A elements, and havingblocked polymerization sites, and containing not more than theequivalent of 19 carbon atoms.

2. The electrolytic cell of claim 1 wherein the polymerization sites inthe acid are blocked by organic groups.

3. The electrolytic cell of claim 2 wherein the blocked acid has thefollowing structural formula:

in which R and R are organic groups;

H is hydrogen;

is oxygen; and

X is selected from the group consisting of nitrogen, phosphorous,arsenic, antimony and bismuth.

4. The electrolytic cell of claim 3 wherein the blocked acid isdimethylphosphinic acid.

5. The electrolytic cell of claim 3 wherein R and R are selected fromthe group consisting of alkyl groups, aryl groups, substituted alkylgroups, and substituted aryl groups.

6. The electrolytic cell of claim 3 wherein R and R are identicalorganic groups.

7. The electrolytic cell of claim 3 wherein R and R are differentorganic groups.

8. The electrolytic cell of claim 2 wherein the blocked acid has thefollowing structural formula:

in which R is an organic group;

O is oxygen;

H is hydrogen; and

Y is selected from the group consisting of carbon, silicon,

germanium, tin, and lead.

9. The electrolytic cell of claim 8 wherein R is selected from the groupconsisting of alkyl groups, aryl groups, substituted alkyl groups, andsubstituted aryl groups.

10. The electrolytic cell of claim 2 wherein the blocked acid has thefollowing structural formula:

in which R and R' are organic groups;

H is hydrogen;

0 is oxygen; and

Z is selected from the group consisting of boron and aluminum.

11. The electrolytic cell of claim 10 wherein R and R' are selected fromthe group consisting of alkyl groups, aryl groups, substituted alkylgroups, and substituted aryl groups.

12. The electrolytic cell of claim 10 wherein R and R are identicalorganic groups.

13. The electrolytic cell of claim 10 wherein R and R are differentorganic groups.

14. The electrolytic cell of claim 2 wherein the anhydride is selectedfrom the group consisting of acetic propionic anhydride, propionicanhydride, isobutyric anhydride, n-butyric anhydride, n-valericanhydride, isovaleric anhydride, dichloroacetic anhydride, caproicanhydride, n-heptoic anhydride, benzoic anhydride, lauric anhydride,chloroacetic anhydride, itaconic anhydride, stearic anhydride, m-toluicanhydride, phenylacetic anhydride, o-cl1lo robenzoic anyhdride,m-chlorobenzoic anhydride, p-toluic anhydride, 4-nitrophthalicanhydride, cinnamic anhydride, phthalic anhydride, o-nitrobenzoicanhydride, a-naphthoic anhydride, m-nitrobenzoic anhydride,3-nitrophthalic anhydride, 1,2-naphthalic anhydride, p-nitrobenzoicanhydride, p-chlorobenzoic anhydride, d-carnphoric anhydride,2,3-naphthalic anhydride, 1,8-naphthalic anhydride, tetrabromophthalicanhydride, and tetraiodophthalic anhydride.

15. The electrolytic cell of claim 2 wherein the anhydride is selectedfrom the group consisting of dimethyl phosphinic anhydride, methylpropyl phosphinic anhydride, ethyl hexyl phosphinic anhydride, dioctylphosphinic anhydride, and octyl decyl phosphinic anhydride.

References Cited UNITED STATES PATENTS 2,830,945 4/ 1958 Keidel 2041 T X3,312,603 4/1967 Wales 204-14 N GERALD L. KAPLAN, Primary Examiner US.Cl. X.R.

